In the prior art, systems have been developed for the continuous dispensing of free flowing materials from storage containers. Typically, such dispensing systems have included a material storage bin having a gated opening in a lower portion thereof through which the material flows into the inlet hopper of a continuous weigh scale, and from which it flows onto the conveyor belt of the latter. The free flowing material which may, for example, be corn, wheat, oats or a granulated fertilizer, flows from the container at a relatively constant rate which is determined by the gate setting, and the conveyor belt is run at the speed necessary to transport the material across the weigh scale at the same rate. Adjustment of the weight of material dispensed per unit time is ordinarily accomplished by adjustmcnt of the setting of the gate, which may be motorized and/or an adjustment of the speed of the conveyor belt.
The weigh scale is capable of keeping track of the flow and its changes, and eventually accounts for these when totaling them. All the weigh scales require continuous monitoring to assure that the desired set weight is maintained and does not drift off because of changes in product bulk-density or flowability.
While systems of the type just described function satisfactorily when the material being handled is of a free-flowing character, they are not suitable for use with non-free flowing materials. In fact, it is not unusual for manufacturers of continuous weigh scales to notify users that such scales are intended for use with free-flowing materials only. Nonfree-flowing materials, rather than flowing freely from a storage container, tend to be arch-forming, i.e., tend to develop a void space over the opening in the bottom of the storage bin. This void space is bounded by an arch-like or curved upper surface which is formed because of interparticle friction, and formation of such void space reduces or stops the material flow from the container. The void space, once started, tends to grow until the weight of the material thereabove exceeds the sum of the combined friction forces maintaining the arch, whereupon the arch collapses and the material again flows.
Certain materials can be free-flowing under one set of conditions and become nonfree-flowing upon a change of conditions such as, for example, a reduction in particle size, an increase in water content, or storage in a larger volume container. Typically, resistance to flow through the discharge opening of a storage container increases when the level of the material in the container is increased, the particle size of the material is reduced, the water content of the material is increased, the coarseness of the particles is increased to thereby increase the friction coefficient thereof, the output rate is reduced as by gate adjustment, or in some cases, when the material is stored for a long period of time. Examples of materials which are of the nonfree-flowing type are powered talc, hydrated lime, starch, urea, fish meal, meat meal, chocolate powder, powdered minerals, kaolin, dried beet pulp, some grass seeds, processed oats, and several animal feeds.
Various means have been used to cope with the problems inherent in the feeding of nonfree-flowing materials from storage containers. A number of such means involve vibratory apparatus which have not been entirely satisfactory because the amount of material fed thereby is subject to considerable variation. My British Pat. No. 1,153,874 discloses a feeder which employs interengaging helical screw-type feeder elements and is operable on a continuous basis to feed a constant volume of nonfree-flowing particulate material from a large storage container. This apparatus has proven to be completely reliable and has found application in a variety of fields, such as in sewerage and water treatment plants, animal feed mills, the pulp and paper industry and the food industry, wherein delivery of a constant volume per unit time is sufficient.
There are many industrial and agricultural applications in which precise weights of materials, some of which are of a nonfree-flowing character, are blended with other ingredients to produce a final product. Because of the problems with the handling of the nonfree-flowing materials, resort has been had to batch weighing of such ingredients and production of the final product on a batch basis. Needless to say, if the same final product could be produced by a continuous, rather than batchwise method, substantial savings of time and expense would result.
In addition to the problem of feeding nonfree-flowing particulate material from a storage container, there is another problem which must be coped with insofar as continuous dispensing of a specific weight of material per unit of time is concerned, and that relates to variations in the bulk density of the material. The bulk density of nonfree-flowing and other materials frequently varies from one delivery to another, and often there may be variations in bulk density within a given delivery. Because of variations in bulk density of material fed by a constant volume type feeder of the type referred to earlier herein, the weight per unit of time of material fed thereby is necessarily subject to variations which usually occur gradually, but are of a magnitude which is unacceptable in applications requiring precise weights of materials.